1. Field of the Invention
The present invention relates to a multistage compressor having a low stage side compressing mechanism and a high stage side compressing mechanism incorporated in a housing.
This application is based on Japanese Patent Application No. 2007-024086, the content of which is incorporated herein by reference.
2. Description of Related Art
Hitherto, various types of multistage compressors used in an air conditioner have been proposed. To give an example thereof, there has been known a multistage compressor that includes a low stage side rotary compressing mechanism below an electric motor provided at the center of a closed housing and functions to inject a gas compressed with the mechanism into the closed housing to supply the gas as an intermediate pressure gas to a high stage side scroll compressing mechanism provided above the electric motor to compress the gas in two stages (see Japanese Unexamined Patent Application Publication No. 5-87074, for instance).
Further, Japanese Unexamined Patent Application Publication No. 2000-54975 discloses a multistage compressor that includes electric motor, low stage side and high stage side rotary compressing mechanisms provided in a closed housing and functions to inject an intermediate pressure gas compressed with the low stage side rotary compressing mechanism to a second sealed chamber provided in the closed housing as well as inject an intermediate pressure gas extracted from a refrigerant circuit side to the second sealed chamber to supply the intermediate pressure injection gas and the intermediate gas compressed with the low stage side rotary compressing mechanism to the high stage side rotary compressing mechanism to compress the gases in two stages.
Further, Japanese Unexamined Patent Application Publication No. 2006-152839 discloses a multistage compressor that uses an R410A refrigerant to supply an intermediate gas compressed with a low stage side rotary compressing element to a high stage side rotary compressing element through a gas pipe and in addition, inject an intermediate gas extracted from a refrigerant circuit side to the gas pipe to compress the gases in two stages. In this compressor, a displacement volume ratio between the low stage side compressing element and the high stage side compressing element is 1:0.65 to 1:0.85.
Furthermore, Japanese Unexamined Patent Application Publication No. 2001-73976 discloses a multistage compressor that injects a part of a CO2 refrigerant gas compressed with a low stage side rotary compressing element to a closed housing and supplies the intermediate pressure CO2 refrigerant gas and the remaining intermediate pressure CO2 refrigerant gas to a high stage side rotary compressing element through a gas pipe to compress the gases in two stages. In this compressor, a capacity ratio between the low stage side compressing element and the high stage side compressing element is 1:0.56 to 1:0.8.
The above two-stage compressors provided with the gas injection circuit as disclosed in Japanese Unexamined Patent Application Publication Nos. 2000-54975 and 2006-152839 can enhance efficiency and capacity of the two-stage compressor owing to an economizer effect of gas injection. However, during a mild-weather season with a low air-conditioning load, the spring or autumn, a high capacity is not necessarily required. To that end, an on/off valve may be provided to a gas injection circuit and closed to turn off the gas injection circuit if an applied load is low. In this case, as in the two-stage compressors as disclosed in Japanese Unexamined Patent Application Publication Nos. 5-87074 and 2001-73976, the compressor functions as a two-stage compressor provided with no gas injection circuit.
However, as understood from the above publications, in the two-stage compressor, an optimum compression ratio between a low stage side compressing element and a high stage side compressing element varies depending on whether a gas injection circuit is provided. Therefore, in a two-stage compressor including a gas injection circuit, if an on/off valve is provided to the gas injection circuit turn on/off the gas injection circuit, a compression ratio between a low stage side and a high stage side is changed, and a preset optimum compression ratio is lost to decrease efficiency and capacity.
This phenomenon might occur also in the case of using a liquid injection circuit that injects an intermediate pressure liquid refrigerant in place of the gas injection circuit.